Why do the rear brake pad get tight when I push the brake pedal?

Must have rear brake SHOES if you replaced the wheel cylinders! Check the park brake cables, may be binding and not releasing correctly.

They are brake shoes if there is a wheel cylinder. Sounds like the springs are weak. Try to bleed the wheel cylinder when they are tight. If cylinder bleeds with a squirt, try to turn the wheel to see if it 'unlocks'. If that works check brake lines or master cylinder. If that doesn't change try spring kit.